linux-kernel - Re: [PATCH v18 05/25] x86/fpu/xstate: Introduce CET MSR and XSAVES supervisor states

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Date:   Mon, 1 Feb 2021 15:14:20 -0800
From:   "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [PATCH v18 05/25] x86/fpu/xstate: Introduce CET MSR and XSAVES
 supervisor states

On 2/1/2021 3:12 PM, Dave Hansen wrote:
> On 2/1/21 3:05 PM, Yu, Yu-cheng wrote:
>>>>
>>>
>>> Wait a sec...  What about *THIS* series?  Will *THIS* series give us
>>> oopses when userspace blasts a new XSAVE buffer in with NT_X86_XSTATE?
>>>
>>
>> Fortunately, CET states are supervisor states.  NT_x86_XSTATE has only
>> user states.
> 
> Ahhh, good point.  You did mention this in the changelog:
> 
>> Control-flow Enforcement Technology (CET) introduces these MSRs:
>>
>>      MSR_IA32_U_CET (user-mode CET settings),
>>      MSR_IA32_PL3_SSP (user-mode shadow stack pointer),
>>
>>      MSR_IA32_PL0_SSP (kernel-mode shadow stack pointer),
>>      MSR_IA32_PL1_SSP (Privilege Level 1 shadow stack pointer),
>>      MSR_IA32_PL2_SSP (Privilege Level 2 shadow stack pointer),
>>      MSR_IA32_S_CET (kernel-mode CET settings),
>>      MSR_IA32_INT_SSP_TAB (exception shadow stack table).
>>
>> The two user-mode MSRs belong to XFEATURE_CET_USER.  The first three of
>> kernel-mode MSRs belong to XFEATURE_CET_KERNEL.  Both XSAVES states are
>> supervisor states.
> 
> This is another great place to add some information about the feature.
> 
> "Both XSAVES states are supervisor states." ...  This means that there
> is no direct, unprivileged access to this state, making it harder for an
> attacker to subvert CET.
> 
> You could also allude to the future ptrace() support here.
> 

I will add that.